The mentioned introduction of the reference air brings moisture into the pressure chamber, which condenses in the interior of the sensor when the temperature falls below the dew point. This can degrade the functioning of the sensor. Such is especially the case, when the air surrounding the sensor has a higher temperature than the medium whose pressure is to be measured.
Even when macroscopic condensation with its undesired side-effects has not yet appeared, still a fraction of the water molecules has already been adsorbed from the gas phase onto the surfaces inside the sensor, with this fraction being a function of the temperature of the relevant surfaces and the adsorption energy between the surface and a water molecule. The greater the adsorption energy, the more water molecules will accumulate on the surfaces inside the sensor.
In the case of ceramic pressure sensors, the described problems are aggravated by manufacturing-related material properties, explained below as follows. Starting point for measuring membrane and platform are so-called green bodies, which are formed from a powdered starting material and a binder, and subsequently sintered, so that sintered bodies are obtained from the green bodies. The sintered bodies must, as a rule, be ground or lapped, and this leads not only to rough surfaces, but also to possible fine hair cracks.
In order to manufacture a pressure sensor from a membrane sintered-body and a platform sintered-body, platform and membrane are soldered or brazed together at their edges with interposition of a spacer, such that the mentioned chamber is formed. The solder or active braze material can itself serve as the spacer. An example of solder is a glass frit, while the active braze material can be e.g. a NiTiZr-alloy, in which the NiZr-fraction is about equal to the NiZr-eutectic; compare U.S. Pat. No. 5,334,344. The soldering or brazing is also referred to as joining.
Before the joining of membrane and platform, electrodes are applied to the surfaces which will be facing one another in the chamber following the joining. These electrodes are composed e.g. of tantalum, compare U.S. Pat. No. 5,050,034, and are usually applied by sputtering. But, with sputtering, it cannot be prevented that, due to so-called under-sputtering, small, mutually electrically separated islands of electrode material occur on the surfaces which are supposed to be kept free of electrode material.